November 6, 2011
Thermal Growth-What’s So Hot About It?
By Stan Riddle
Engineering and technical people often use technical phrases that sound complicated. Thermal growth is one of those terms. It sounds like it could be an engineering marvel – one that would help us perform complicated alignments of high temperature, high speed machines. So, here’s the “secret” of thermal growth.
When metal gets hotter, it gets bigger.
Just how big is based on three things:
- (What it is) The coefficient of linear thermal expansion, C
- (How hot it gets) The differential temperature of the material being heated, ∆T
- (How tall it is) The linear dimension of the material being calculated, L
It is calculated using the following formula:
C x ∆T x L = amount of thermal growth
Still sounding complicated? Try this comparison.
12 inches of these commonly used materials, heated to 100°F above room temperature, yields the following growth:
Cast Iron 0.007”
Mild Steel 0.008”
12 in. of cast iron, heated 100° over room temperature = 0.007”. But we have 24”, so:
0.007 x 2 (12 inches twice) = 0.014”.
It sounds right, but not so fast!
Thermal growth is not calculated by the fluid temperature, but by the pump casing temperature. That’s what’s holding the pump shaft up. And it almost never heats evenly from the bottom to the split line. So you must either:
- Make an estimated calculation of the thermal growth by sampling the temperature in several places, and getting an average temperature,
- Use the HotCheck function on your laser alignment tool (if available), to get a comparison between the hot and cold alignment values,
- Use a dynamic measuring process, such as Fixturlaser’s OL2R (off line to running) system, to get an extremely accurate measurement of thermal growth, which would also include dynamic factors, such as piping changes.
Thermal growth is not always a factor in aligning machinery, but if your machine is:
- or must be aligned to a very close tolerance,
thermal growth must be factored as part of the alignment process.
How do you determine thermal growth factors and how they should be used?